1. Field of the Invention
The invention relates to pulse width modulation control for a current in a coil in a motor.
2. Description of the Background Art
Conventionally, it has been known that a current in a coil in a motor is controlled by using pulse width modulation control. The term pulse width modulation is also referred to as “PWM” below. A direct current motor control apparatus that changes a short cycle of a PWM signal to a long cycle in a region in which a duty cycle of the PWM signal is high, is known as one of technologies relating to the PWM control of a motor.
When a duty cycle is high, close to “100%,” in the PWM control, there is a case where an OFF period of a signal controlling a switching element is shorter than a response time period required for the switching element and/or a driving circuit thereof. In such a case of the high duty cycle, a current of a controlled object cannot be cut off in the OFF period in which the current needs to be cut off. Thus, an energy loss may occur by switching. Similarly, when a duty cycle is low, close to “0%,” a loss of switching may occur because the target current does not flow in an ON period in which the target current needs to flow. The term “loss by switching” collectively refers to both problems.
With reference to FIG. 1, an example of a loss by switching in a case of using an n-channel MOSFET (metal-oxide-semiconductor field-effect transistor), is explained. An upper drawing in FIG. 1 illustrates a timeline chart showing an example of the MOSFET switching control signal. A lower drawing in FIG. 1 illustrates a timeline chart showing an example of a gate voltage of the MOSFET.
Symbols Ton and Toff represent the ON period and the OFF period respectively. For example, when the OFF period Toff is shorter than a response time of a driving circuit of the MOSFET, the gate voltage of the MOSFET does not become less than an on-state voltage Vt during the OFF period Toff. As a result, since the MOSFET is not turned off, the target current is not cut off.
In a case of the PWM control, when the OFF period or the ON period of the pulse defined by a duty cycle is shorter than a response time of the switching element and/or a driving circuit thereof, a response may not catch up to a switching operation. Thus there is a problem where control efficiency decreases.